1. Field of the Invention
The present invention relates to a method for forming a microstructure by dry etching and particularly to a method for forming a micro groove structure excellent in shape controllability.
2. Related Art
Optical elements having microstructures shape-controlled on scales of from the order of microns to the order of nanometers have been developed with the advance of the field of information communications in recent years. Functional elements of similar microstructures have been developed also in the biotechnological and medical field as well as the information communication field.
At present, lithography technology is a typical micromachining technique. This is a technique for forming a micro concavoconvex shape in a substrate material by etching after forming a desired pattern by irradiating a photosensitive resist with energy wave such as ultraviolet rays, electron beams or X-rays and masking the substrate material with the resist per se or transferring the pattern onto a high durable material such as metal and masking the substrate material with the high durable material.
On the other hand, sophistication of the micro shape formed in the substrate, increase of the aspect ratio in the concavoconvex shape, diversification of the material to be etched, etc. are required of an etching technique. Dry etching using plasma has begun to be frequently used as a technique which meets such requirements of more accurate etching.
As the mask pattern is sophisticated and made micro as described above, abrasion of the masking material at etching, however, becomes an issue. Particularly when a deep groove needs to be formed in the substrate, there occurs a problem that the groove depth allowed to be etched is decided by abrasion of the masking material.
Although improvement of selective ratio indicating the etching speed ratio between the masking material and the substrate material can be achieved to a certain degree by selection of the masking material, etching gas, plasma condition, etc., the improvement achieved has a limit when it depends on physical and chemical properties of the material.
Although the durability of the mask can be improved when the mask per se is made thick, the mask needs to be patterned by photolithography or the like in advance. The mask allowed to be machined from a photo resist finite in thickness has a limit in thickness. In addition, provision of the mask as a thicker film than required brings a problem that micro pattern accuracy is spoiled.